Acrylamide has hitherto been prepared by a so-called catalytic hydration process which comprises reacting acrylonitrile with water in the presence of a catalyst, such as a copper catalyst.
An aqueous acrylamide solution prepared by the catalytic hydration process tends to undergo coloration or become turbid immediately after the preparation thereof, or with the passage of time, due to trace amounts of impurities, such as decomposition products of a stabilizer present in the starting acrylonitrile, substances eluted from the catalyst used, by-products, and the like.
The crude aqueous acrylamide solution containing such impurities should be subjected to a purification step to remove the color or turbidity before it is commercially presented in the form of an aqueous solution or crystals. It has been generally considered preferable to carry the purification by passing the aqueous solution through a column packed with activated carbon, particularly granular activated carbon. However, acrylamide is very apt to be polymerized around activated carbon, thus causing obstruction of the column. As a result, the advantages of the granular activated carbon cannot be fully utilized. In an attempt to prevent polymerization of acrylamide, a method has been proposed of incorporating cupric ion in the activated carbon in advance, as disclosed in Japanese Patent Publication No. 28608/76 (corresponding to U.S. Pat. No. 3,923,741).
In recent years, a process for preparing acrylamide by direct hydration of acrylonitrile using microorganisms capable of hydrating nitriles has been proposed, as described, e.g., in Japanese patent application Ser. No. (OPI) 86186/76 (corresponding to U.S. Pat. No. 4,001,081) and Japanese Patent Publication No. 17918/81 (corresponding to U.S. Pat. No. 4,248,968) (the term "OPI" used herein means an "unexamined published application"). According to this microbiological process, if the acrylamide concentration is increased, pigments and traces of impurities tend to be extracted from the microorganism to enter into the aqueous solution. Therefore, it is desirable to purify the aqueous solution obtained by this process by treating with activated carbon similarly as in the case of the aforesaid catalytic hydration process.
However, when the acrylamide aqueous solution obtained by the microbiological process is purified using activated carbon in which cupric ion has been incorporated for the purpose of preventing polymerization of acrylamide in accordance with the conventional technique, the cupric ion tends to be extracted into the aqueous solution, resulting in not only reduction of cupric ions adsorbed on the activated carbon to readily cause polymerization of acrylamide around the activated carbon, but also incorporation of cupric ions that were not formerly present in the crude aqueous solution in the purified solution. The extraction or dissolution of cupric ions in the acrylamide aqueous solution is believed related to a copper ion equilibrium between activated carbon and the aqueous solution. An aqueous acrylamide solution containing a cupric ion even in a trace amount is unsuitable as a monomer for obtaining high molecular weight polymers, and is, therefore, required to be further purified by ion-exchange resins.